A conventional gas turbine is configured to predetermine an air flow rate and a fuel flow rate to be sent to a combustor on the basis of a power generator output, an ambient temperature, humidity, and the like, and to perform operation based on the determined flow rate values. However, there is a possibility that fuel composition, the fuel flow rate, and the air flow rate in practice are deviated from those at the time of planning or at the time of test run adjustment due to variation in the composition or the heat capacity of the fuel gas to be supplied to the gas turbine or to secular changes such as deterioration in performance of a compressor or filter clogging, for example. Attributed to such deviation, there arises a risk of deterioration in combustion stability and eventual occurrence of combustion oscillation which constitutes a major obstacle to operation of the gas turbine. Accordingly, from the viewpoints of equipment protection and improvement in the operating rate, there is a strong demand for suppressing and avoiding occurrence of this combustion oscillation as much as possible.
Meanwhile, the fuel gas stored in a fuel tank, for instance, is supplied to the combustor of the gas turbine. However, inside this fuel tank for storing the fuel gas, heavy molecules out of components constituting the fuel gas go down while light molecules go up over time. As a result, the fuel composition of the fuel gas supplied to the gas turbine fluctuates in response to the residual amount of the fuel inside the fuel tank or to the time, whereby such a difference in the fuel composition generates a change in a combustive action in the combustor which may lead to reduction in the combustion stability. Moreover, the fuel composition fluctuates largely in the case of using blast furnace gas as the fuel, for example, and similar reduction in the combustion stability may occur.
In response, a gas turbine controlling method configured to control a fuel flow rate and an air flow rate in a combustor in response to an amount of variation in a fuel calorific value based on a fuel composition has been disclosed as a gas turbine controlling method for maintaining this combustion stability (see Patent Document 1). According to the gas turbine controlling method disclosed in this patent document, adjustment for the amount of variation in the fuel calorific value is performed by setting up a bias coefficient corresponding to the amount of variation in the fuel calorific value based on the fuel composition, and by performing addition after multiplying operation amounts of a fuel flow rate and an air flow rate by the set bias coefficient.    Patent Document 1: Japanese Patent Application Laid-open Publication No. 5 (1993)-187271